Update dotnet samples to .NET 10 with latest NuGet packages

[Copilot]

Upgrades all 24 .NET projects under samples/durable-task-sdks/dotnet/ to target net10.0 and bumps all dependencies to their latest compatible versions.

Target framework

• 18 projects updated from net8.0 → net10.0
• 5 projects already on net10.0 had stale package versions updated
• 1 project (HumanInteraction) already fully up to date — used as reference

Package versions

Package	Before	After
Microsoft.DurableTask.*.AzureManaged	1.17.1	1.21.0
Microsoft.DurableTask.Generators	1.0.0-preview.1	2.0.0-preview.1
Microsoft.DurableTask.ScheduledTasks	1.10.0-preview.1	1.21.0-preview.1
Microsoft.Extensions.*	8.x / 9.x	10.0.3
Azure.Identity	1.13.1–1.17.0	1.17.1
Grpc.Net.Client	2.67.0	2.76.0
Aspire.AppHost.Sdk	13.0.2	13.1.2
Microsoft.AspNetCore.OpenApi	10.0.0	10.0.3

Central package management

• Agents/PromptChaining/Directory.Packages.props updated: DurableTask SDK → 1.21.0, Azure.AI.Agents.Persistent → 1.1.0, Azure.Core → 1.51.1, all Extensions → 10.0.3

CI workflow

• Updated comment annotations from (net8.0) to (net10.0) for DTS samples
• Retained .NET 8 SDK setup — still needed by durable-functions/dotnet and scenarios/ samples

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